obj Import .mtl File
Please check if the name in the code (mtllib Name.mtl) from the OBJ file is the same as the MTL file name.

If it's not please tweak the code or change the name of the MTL file.
This issue happened to me too when i realised the OBJ file doesn't have the same name
in the code.

If this doesn't work and/or the filename was the same then there
is probably an issue with the code. If the code doesn't match the pictures name's then you schould modify the names.

~Randy